U.S. patent number 7,944,116 [Application Number 12/160,969] was granted by the patent office on 2011-05-17 for drive circuit.
This patent grant is currently assigned to Dyson Technology Limited. Invention is credited to Stephen James Causier.
United States Patent |
7,944,116 |
Causier |
May 17, 2011 |
Drive circuit
Abstract
A drive circuit for a high-frequency agitation source includes a
signal generator generating a train of low voltage square-wave
pulses at a drive frequency, a booster including a boost inductor
generating a back EMF and configured to produce a high-voltage
pulse train in response to the low-voltage square-wave pulse train
and a filter producing a drive signal having a pre-determined
harmonic of the drive frequency, the drive signal being used to
drive the high-frequency agitation source. The drive circuit is
particularly suitable for use with piezoelectric crystals.
Inventors: |
Causier; Stephen James
(Wiltshire, GB) |
Assignee: |
Dyson Technology Limited
(Wiltshire, GB)
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Family
ID: |
35998139 |
Appl.
No.: |
12/160,969 |
Filed: |
December 11, 2006 |
PCT
Filed: |
December 11, 2006 |
PCT No.: |
PCT/GB2006/004606 |
371(c)(1),(2),(4) Date: |
August 06, 2008 |
PCT
Pub. No.: |
WO2007/083075 |
PCT
Pub. Date: |
July 26, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20100236092 A1 |
Sep 23, 2010 |
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Foreign Application Priority Data
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Jan 17, 2006 [GB] |
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0600868.4 |
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Current U.S.
Class: |
310/317;
310/321 |
Current CPC
Class: |
H02M
7/537 (20130101); B06B 1/023 (20130101); B05B
17/0607 (20130101); H01L 41/042 (20130101) |
Current International
Class: |
H01L
41/09 (20060101) |
Field of
Search: |
;310/317,318,314 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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10353835 |
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Jun 2005 |
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DE |
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0767504 |
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Apr 1997 |
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EP |
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WO-01/29957 |
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Apr 2001 |
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WO |
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WO 2005/080793 |
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Sep 2005 |
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WO |
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Other References
GB Search Report directed towards counterpart application No.
GB0600868.4; 1 page. cited by other .
International Search Report directed towards counterpart
application No. PCT/GB2006/004606; 3 pages. cited by other .
International Preliminary Report on Patentability directed towards
counterpart application No. PCT/GB2006/004606; 5 pages. cited by
other.
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Primary Examiner: Rosenau; Derek J
Attorney, Agent or Firm: Morrison & Foerster LLP
Claims
The invention claimed is:
1. A hand dryer, comprising: a nebulizer, the nebulizer comprising
a drive circuit for a high-frequency agitation source, the drive
circuit comprising: a signal generating stage generating a train of
low voltage square-wave pulses at a drive frequency, a boost stage
including a boost inductor for generating a back EMF, the boost
stage being configured to produce a high-voltage pulse train in
response to the low-voltage square-wave pulse train, and a filter
stage producing from the high-voltage pulse train a drive signal
having a pre-determined harmonic of the drive frequency, the drive
signal being used to drive the high-frequency agitation source.
2. The drive circuit of claim 1, wherein the high-frequency
agitation source is a piezoelectric crystal.
3. The drive circuit of claim 1, wherein the filter stage comprises
a low-pass filter.
4. The drive circuit of claim 3, wherein the filter stage comprises
an inductor in series with the high-frequency agitation source and
a capacitor in parallel with the high-frequency agitation
source.
5. The drive circuit of claim 1, further comprising a DC power
source and a power rail supplied by the DC power source.
6. The drive circuit of claim 5, wherein the boost inductor is
connected in parallel between the power rail of the DC power source
and ground.
7. The drive circuit of claim 5, wherein the boost stage further
comprises a switch that switches the power rail on or off in
response to the train of low-voltage square-wave pulses.
8. The drive circuit of claim 1, wherein the drive signal frequency
produced by the drive circuit is in the region of 1.5-2 MHz.
9. The drive circuit of claim 1, wherein the boost inductor
generates a back EMF greater than 50 V.
Description
REFERENCE TO RELATED APPLICATIONS
This application is a national stage application under 35 USC 371
of International Application No. PCT/GB06/004606, filed Dec. 11,
2006, which claims the priority of United Kingdom Application No.
0600868.4, filed Jan. 17, 2006, the contents of both of which prior
applications are incorporated herein by reference.
FIELD OF THE INVENTION
The invention relates to a drive circuit for a high-frequency
agitation source. Particularly, the invention relates to a drive
circuit for a piezoelectric crystal.
BACKGROUND OF THE INVENTION
Piezoelectric crystals are well known in the art and are used for a
number of purposes. Piezoelectric motors, transformers and linear
drives are common. An important use for a piezoelectric crystal is
in nebulisation. There are many cases where a fine mist of a
substance is required without the application of heat. One example
of this is a medical nebuliser, wherein a pharmaceutical compound
is nebulised by a piezoelectric crystal in order to be inhaled by a
patient. Another use for nebulisers is in the field of water
dispersal such as garden water features. In order to disperse a
dispersal agent effectively, a high voltage, high frequency drive
source is required. Typically, a piezoelectric crystal for use in
nebulisation is driven at its resonance frequency. This frequency
varies between piezoelectric crystals, however it is usually in the
region of 1.6-1.7 MHz.
SUMMARY OF THE INVENTION
Drive circuits for piezoelectric crystals are well known in the
art. A simple way of generating such a high frequency signal is
through the use of a transistor circuit. However, if this is done,
a high voltage amplifier or a transformer is required to generate
the peak to peak voltages needed to drive a piezoelectric crystal.
Typically, these voltages are in the region of 100-150 V.
Transformers are the most commonly used components for this
purpose. However, they are often bulky and expensive.
A further requirement for an electronic device that will use a
mains power supply is that Electromagnetic Compatibility Standards
(EMC) have to be met. These standards define an acceptable level
for the harmonic content in the current which electrical equipment
draws from a mains AC supply, as well as an acceptable level of
voltage distortion. A high-voltage square wave signal may contain
an unacceptable level of harmonic content both for efficient
driving of a piezoelectric crystal and for meeting the required
standards of harmonic content. A common way of solving this problem
is to pass the signal through a low-pass filter. If the low-pass
filter is tuned to the fundamental driving frequency of the
piezoelectric crystal, higher order harmonics can be filtered out,
leaving only the fundamental frequency to drive the piezoelectric
crystal. Often, a low-pass filter is also used to give a voltage
gain. However, in order to drive a piezoelectric crystal at
resonance, a relatively high quality factor is required. In order
to achieve this with a low-pass filter such as an LC circuit, the
capacitances of the system in which the LC circuit is located needs
to be constant. However, the capacitance of wiring and the
piezoelectric crystal itself may vary with temperature, age,
condition and use. Therefore, this often makes an LC circuit
unsuitable for driving a piezoelectric crystal at the precise
resonant frequency.
The invention provides a drive circuit for a high-frequency
agitation source, the drive circuit comprising signal generating
means for generating a train of low voltage square-wave pulses at a
drive frequency, boost means including a boost inductor for
generating a back EMF, the boost means being arranged to produce a
high-voltage pulse train in response to the low-voltage square-wave
pulse train and filter means for producing from the high-voltage
pulse train a drive signal having a pre-determined harmonic of the
drive frequency, the drive signal being used to drive the
high-frequency agitation source. Using the back EMF from an
inductor to generate a high-voltage pulse train avoids the use of
bulky and expensive transformers.
Preferably, the high-frequency agitation source is a piezoelectric
crystal.
Advantageously, the filter means comprises a low-pass filter which
includes an inductor in series with the high-frequency agitation
source and a capacitor in parallel with the high-frequency
agitation source.
The invention provides a simple and cost-effective circuit which is
able to generate a high-voltage, high-frequency, clean sine wave
signal to drive a piezoelectric crystal. The invention is
particularly suitable to drive a nebuliser for use in a hand
dryer.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention will now be described with reference
to the accompanying drawings, in which:
FIG. 1 is a circuit diagram of a drive circuit according to the
invention;
FIG. 2 is a graph showing an input signal S2 to a boost stage and a
high-voltage output signal S3 from the boost stage;
FIG. 3 is a graph showing the cut off frequency of a filter
stage;
FIG. 4a is a graph showing the high-voltage output S3 input to the
filter stage and an output waveform S4 outputted from the filter
stage;
FIG. 4b is an oscilloscope trace showing an actual output waveform
S4 as supplied to the piezoelectric crystal;
FIG. 5 shows a fast fourier transform of the output waveform S4
illustrating the harmonic components of the waveform S4; and
FIG. 6 shows a hand dryer incorporating a nebuliser driven by the
drive circuit of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a drive circuit according to the invention. The drive
circuit is powered by a DC power source (not shown). The DC power
source originates from an AC/DC converter powered by a mains
electricity supply. The drive circuit comprises three stages: a
signal generation stage 1, a boost stage 2 and a filter stage 3.
The first stage is the signal generation stage 1. The signal
generation stage 1 comprises a microprocessor unit MP1 for
generating a synchronisation signal at, say, 1660 KHz. The
microprocessor unit MP1 is supplied at low voltage, for example 3.3
V. This microprocessor unit MP1 includes a phase-locked loop for
multiplying the synchronisation signal to the required drive
frequency. The output from the microprocessor unit MP1 is connected
to a pair of complementary push-pull Metal Oxide Semiconductor
Field Effect Transistors (MOSFETs) TR1, TR2. MOSFET TR1 is a low
power p-channel MOSFET, and MOSFET TR2 is a low power n-channel
MOSFET. The pair of MOSFETs TR1, TR2 provide a push-pull output
drive. The push-pull arrangement of the MOSFETs TR1, TR2 is
required to sink and source the gate charge and minimise switching
losses. The output from the push-pull MOSFETs TR1, TR2 is connected
to the gate of a power MOSFET TR3. The power MOSFET TR3 is supplied
by a 5 V power rail. The source and drain of the power MOSFET TR3
form part of the boost stage 2 and act as a switch in the boost
stage 2.
The boost stage 2 comprises an inductor L1, the source/drain of the
power MOSFET TR3 and a capacitor C1. The capacitor C1 is connected
in parallel across the source/drain of the power MOSFET TR3. These
components are connected between the 24 V and ground power rails of
the power source. The inductor L1 has an inductance of 15 .mu.H and
the capacitor C1 has a capacitance of 1 nF.
Connected across the inductor L1 is the filter stage 3. The filter
stage 3 comprises a low pass filter. The low-pass filter includes
an inductor L2 in series with the boost stage 2, and a capacitor C2
in parallel with the boost stage 2. The capacitance of capacitor C2
and the inductance of the inductor L2 are selected such that the
resonant frequency of the low-pass filter is approximately equal to
the drive frequency of the piezoelectric crystal. The capacitor C2
has a capacitance of 2.2 nF and the inductor L2 has an inductance
of 4.7 .mu.H. FIG. 3 shows the attenuation characteristics of the
filter stage. These values are chosen in order to provide a 3 dB
roll off frequency of approximately 1.6 MHz. Expressed another way,
the resonant frequency of the filter stage 3 is centred on the
drive frequency of the piezoelectric crystal according to the
relationship f.sub.0=1/(2.pi. LC) where L is the inductance of the
inductor L2 and C is the capacitance of the capacitor C2. Connected
across the output from the filter stage 3 is a piezoelectric
crystal P1.
In operation, the microprocessor generates a 1660 KHz
synchronisation signal. The phase-locked loop multiplies the
synchronisation signal by 1024 to generate a drive signal S1 close
to 1.7 MHz. The drive signal S1 from the microprocessor unit MP1 is
then supplied to the complementary push-pull transistor driver. The
MOSFETs TR1, TR2 of the push-pull drive generate a square-wave
signal S2 which is supplied to the power MOSFET TR3.
The square-wave signal S2 switches the power MOSFET TR3 on or off
depending upon whether the square-wave signal S2 is high or low.
When the square-wave signal S2 is high, the power MOSFET TR3 is
switched on, the source/drain of the power MOSFET TR3 conducts and
completes the circuit between the 24 V power rail and ground. When
this happens, the inductor L1 begins to charge. When the
square-wave signal S2 returns to a low state, the power MOSFET TR3
is switched off. This generates a large rate of change of current
in the boost stage 2. The magnetic field established in the
inductor L1 during the on phase of the MOSFET TR3 attempts to
resist the change in current. This generates a large back EMF in
the inductor L1 which produces a high-voltage output signal S3. The
high-voltage output signal S3 is shown in FIG. 2. The high-voltage
output signal S3 consists of a series of peaks which correspond to
the back emf generated by the inductor L1. The timing of the
leading edges of the peaks corresponds to the timing of the
trailing edges of the square-wave signal S2. The high-voltage
output signal S3 has the same duty cycle as the square-wave signal
S2. The peak amplitude of the high-voltage output signal S3 is in
the region of 90 V. The peak amplitude of the high-voltage output
signal S3 is limited by the capacitor C1. The capacitor C1 spreads
the energy released by the inductor L1 over a greater time period,
reducing the maximum peak voltage generated. This is required to
protect the power MOSFET TR3 from damage.
The high-voltage output signal S3 has a high voltage and a pulse
period equal to the inverse of the drive frequency. However, it is
not a clean signal. By this is meant that the high-voltage output
signal S3 comprises a number of different frequencies in addition
to the fundamental frequency. Any waveform or pulse train can be
expressed as a superposition of sine waves of different harmonic
frequencies. The high-voltage output signal S3 comprises a large
number of unwanted harmonic frequencies. These harmonic frequencies
are undesirable because they may affect the operation of the
piezoelectric crystal and generate a large amount of unwanted
harmonic distortion.
In order to remove the unwanted higher harmonic frequencies from
the high-voltage output signal S3 and leave only the fundamental
frequency, the filter stage 3 is used. The filter stage 3 removes
the higher order harmonics present in the high-voltage output
signal S3, and the output S4 from the filter stage 3 is a clean
sine wave with a peak-to-peak voltage of 100-140 V and a drive
frequency of 1.7 MHz. FIG. 4a shows a schematic drawing of the
input waveform of the high-voltage output S3 and the output
waveform S4. FIG. 4b shows an actual output waveform S4 output from
the filter stage 3 as "seen" by the piezoelectric crystal P1. The
waveform is a sine-wave at the fundamental frequency of
approximately 1.7 MHz. FIG. 5 shows a fast Fourier transform of
this waveform. The X-axis shows the frequency (in MHz) and the
Y-axis shows the strength of the harmonic components (in units of
dBVrms). The figure illustrates that the low pass filter
successfully removes the majority of the unwanted harmonic
frequencies. A component of the second harmonic still remains,
however it is attenuated such that the circuit meets EMC
requirements. The output S4 is then used to drive the piezoelectric
crystal at a frequency of approximately 1.7 MHz.
The above-described embodiment of the invention is a low-cost
circuit for generating a clean, high-voltage, high-frequency
sinusoidal waveform from a DC source. The invention may be used in
any situation where a high frequency agitation source is required
to be driven cheaply and effectively. The low component count of
the circuit and the absence of a transformer also reduces the
physical size of the circuit. This is of benefit to applications
where size is a crucial factor, for example, household appliances
or medical devices.
The above-described embodiment of the invention is particularly
suited for use in a hand dryer such as that shown in FIG. 6. The
hand dryer 100 includes a cavity 110. The cavity 110 is open at its
upper end 120 and the dimensions of the opening are sufficient to
allow a user's hands (not shown) to be inserted easily into the
cavity 110 for drying. A high-speed airflow is generated by a motor
unit having a fan (not shown). The high-speed airflow is expelled
through two slot-like openings 130 disposed at the upper end 120 of
the cavity 110 to dry the user's hands. A drain (not shown) for
draining the water removed from a user's hands from the cavity 110
is located at the lower end of the cavity 110. A nebuliser 140 is
located downstream of the drain. The nebuliser 140 is shown
partially removed from the hand dryer 100 in FIG. 6. The nebuliser
140 is partially cut away to show the location of the
above-described drive circuit 150. The nebuliser 140 includes a
collector (not shown) for collecting waste water and a
piezoelectric crystal (not shown) for nebulising the waste water.
The piezoelectric crystal is driven by the drive circuit 150. The
low component count and low cost of the drive circuit means that it
is smaller, cheaper to manufacture and less likely to fail. This
means that the size of the hand dryer can be reduced, the
reliability of the hand dryer can be improved and the cost of
maintenance is reduced.
It will be appreciated that the invention is not limited to the
embodiment illustrated in the drawings. The magnitude and frequency
of the drive source may be varied depending upon the required
application. For example, it is common to drive a piezoelectric
crystal at a range of frequencies. However, it is most common to
drive a piezoelectric crystal at, or close to, its resonant
frequency. For most piezoelectric crystals this frequency lies in
the range between 1.5 to 2 MHz.
Further, the physical quantities of the described electronic
components also may be varied in value. This could be done, for
example, to change the resonant point of the filter stage, or to
increase or decrease the back EMF generated by the boost inductor.
However, it is desirable that the back EMF generated by the boost
inductor is greater than 50 V.
There need not be only one low-pass LC filter. The filter stage 3
may comprise two LC filters in series to attenuate better the
higher harmonic frequencies. Further, other forms of signal
generator could be used. What is important is that an inductor is
used to generate a back EMF to amplify a pulse train, and this
signal is then converted into a single-frequency sine wave using a
filter.
* * * * *